1. Field of the Invention
The present invention relates to a thermal head and a printer including the thermal head.
2. Description of the Related Art
There has been conventionally known a thermal head used in a thermal printer to effect printing onto a thermosensitive recording medium by selectively driving a plurality of heating elements based on printing data (see, for example, patent document JP 2007-83532 A).
As a method for achieving a reduction in power consumption by improving thermal efficiency of a heating resistor in a thermal head, there has been known a method in which a hollow portion is formed in a region opposing the heating resistor. By allowing the hollow portion to function as a heat insulating layer having a low thermal conductivity, and reducing an amount of heat propagated and dissipated from the heating resistor to a support substrate, efficiency of energy used for printing may be improved.
Such a thermal head having a hollow portion is formed by providing a silicon substrate (lower plate substrate) with a concave portion by etching or laser processing, bonding a glass thin plate (upper plate substrate) serving as a heat accumulating layer onto the silicon substrate, and then processing the upper plate substrate to a desired thickness by polishing.
In such a thermal head having a hollow portion, when the thickness of an upper plate substrate which supports thereon a heating resistor is reduced to enlarge the hollow portion, heat insulating performance increases so that the thermal efficiency of the thermal head is improved. On the other hand, when the thickness of the upper plate substrate is reduced, the strength thereof decreases. Accordingly, in order to ensure a strength required to support the heating resistor while maintaining the thermal efficiency, thickness control over the upper plate substrate is important. Therefore, it is necessary to accurately perform the polishing of the upper plate substrate.
However, in the method disclosed in JP 2007-83532 A, when the two glass plates are bonded together, and then the upper plate substrate is polished to obtain a glass thin plate having a desired thickness, it is inevitable to measure the total thickness of the upper plate substrate and the lower plate substrate in a bonded state. Accordingly, variations in the thickness of the lower plate substrate are involved in the thickness of the upper plate substrate to be measured, which results in a problem of a reduction in accuracy of measuring the thickness of the upper plate substrate. In addition, the thickness is measured with a measurement device by pinching an outer edge portion of the substrate including the upper and lower plate substrates bonded together, and hence a problem arises that only the thickness of the outer edge portion of the substrate may be measured.